PCREPATTERN(3)PCREPATTERN(3)NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions that are supported
by PCRE are described in detail below. There is a quick-reference syn‐
tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
semantics as closely as it can. PCRE also supports some alternative
regular expression syntax (which does not conflict with the Perl syn‐
tax) in order to provide some compatibility with regular expressions in
Python, .NET, and Oniguruma.
Perl's regular expressions are described in its own documentation, and
regular expressions in general are covered in a number of books, some
of which have copious examples. Jeffrey Friedl's "Mastering Regular
Expressions", published by O'Reilly, covers regular expressions in
great detail. This description of PCRE's regular expressions is
intended as reference material.
This document discusses the patterns that are supported by PCRE when
one its main matching functions, pcre_exec() (8-bit) or
pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has alternative
matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which
match using a different algorithm that is not Perl-compatible. Some of
the features discussed below are not available when DFA matching is
used. The advantages and disadvantages of the alternative functions,
and how they differ from the normal functions, are discussed in the
pcrematching page.
SPECIAL START-OF-PATTERN ITEMS
A number of options that can be passed to pcre_compile() can also be
set by special items at the start of a pattern. These are not Perl-com‐
patible, but are provided to make these options accessible to pattern
writers who are not able to change the program that processes the pat‐
tern. Any number of these items may appear, but they must all be
together right at the start of the pattern string, and the letters must
be in upper case.
UTF support
The original operation of PCRE was on strings of one-byte characters.
However, there is now also support for UTF-8 strings in the original
library, an extra library that supports 16-bit and UTF-16 character
strings, and a third library that supports 32-bit and UTF-32 character
strings. To use these features, PCRE must be built to include appropri‐
ate support. When using UTF strings you must either call the compiling
function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the
pattern must start with one of these special sequences:
(*UTF8)
(*UTF16)
(*UTF32)
(*UTF)
(*UTF) is a generic sequence that can be used with any of the
libraries. Starting a pattern with such a sequence is equivalent to
setting the relevant option. How setting a UTF mode affects pattern
matching is mentioned in several places below. There is also a summary
of features in the pcreunicode page.
Some applications that allow their users to supply patterns may wish to
restrict them to non-UTF data for security reasons. If the
PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not
allowed, and their appearance causes an error.
Unicode property support
Another special sequence that may appear at the start of a pattern is
(*UCP). This has the same effect as setting the PCRE_UCP option: it
causes sequences such as \d and \w to use Unicode properties to deter‐
mine character types, instead of recognizing only characters with codes
less than 128 via a lookup table.
Disabling auto-possessification
If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops
PCRE from making quantifiers possessive when what follows cannot match
the repeated item. For example, by default a+b is treated as a++b. For
more details, see the pcreapi documentation.
Disabling start-up optimizations
If a pattern starts with (*NO_START_OPT), it has the same effect as
setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
time. This disables several optimizations for quickly reaching "no
match" results. For more details, see the pcreapi documentation.
Newline conventions
PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (line‐
feed) character, the two-character sequence CRLF, any of the three pre‐
ceding, or any Unicode newline sequence. The pcreapi page has further
discussion about newlines, and shows how to set the newline convention
in the options arguments for the compiling and matching functions.
It is also possible to specify a newline convention by starting a pat‐
tern string with one of the following five sequences:
(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences
These override the default and the options given to the compiling func‐
tion. For example, on a Unix system where LF is the default newline
sequence, the pattern
(*CR)a.b
changes the convention to CR. That pattern matches "a\nb" because LF is
no longer a newline. If more than one of these settings is present, the
last one is used.
The newline convention affects where the circumflex and dollar asser‐
tions are true. It also affects the interpretation of the dot metachar‐
acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it
does not affect what the \R escape sequence matches. By default, this
is any Unicode newline sequence, for Perl compatibility. However, this
can be changed; see the description of \R in the section entitled "New‐
line sequences" below. A change of \R setting can be combined with a
change of newline convention.
Setting match and recursion limits
The caller of pcre_exec() can set a limit on the number of times the
internal match() function is called and on the maximum depth of recur‐
sive calls. These facilities are provided to catch runaway matches that
are provoked by patterns with huge matching trees (a typical example is
a pattern with nested unlimited repeats) and to avoid running out of
system stack by too much recursion. When one of these limits is
reached, pcre_exec() gives an error return. The limits can also be set
by items at the start of the pattern of the form
(*LIMIT_MATCH=d)
(*LIMIT_RECURSION=d)
where d is any number of decimal digits. However, the value of the set‐
ting must be less than the value set (or defaulted) by the caller of
pcre_exec() for it to have any effect. In other words, the pattern
writer can lower the limits set by the programmer, but not raise them.
If there is more than one setting of one of these limits, the lower
value is used.
EBCDIC CHARACTER CODES
PCRE can be compiled to run in an environment that uses EBCDIC as its
character code rather than ASCII or Unicode (typically a mainframe sys‐
tem). In the sections below, character code values are ASCII or Uni‐
code; in an EBCDIC environment these characters may have different code
values, and there are no code points greater than 255.
CHARACTERS AND METACHARACTERS
A regular expression is a pattern that is matched against a subject
string from left to right. Most characters stand for themselves in a
pattern, and match the corresponding characters in the subject. As a
trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the PCRE_CASELESS option), letters are
matched independently of case. In a UTF mode, PCRE always understands
the concept of case for characters whose values are less than 128, so
caseless matching is always possible. For characters with higher val‐
ues, the concept of case is supported if PCRE is compiled with Unicode
property support, but not otherwise. If you want to use caseless
matching for characters 128 and above, you must ensure that PCRE is
compiled with Unicode property support as well as with UTF support.
The power of regular expressions comes from the ability to include
alternatives and repetitions in the pattern. These are encoded in the
pattern by the use of metacharacters, which do not stand for themselves
but instead are interpreted in some special way.
There are two different sets of metacharacters: those that are recog‐
nized anywhere in the pattern except within square brackets, and those
that are recognized within square brackets. Outside square brackets,
the metacharacters are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character
class". In a character class the only metacharacters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the metacharacters.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by
a character that is not a number or a letter, it takes away any special
meaning that character may have. This use of backslash as an escape
character applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the
pattern. This escaping action applies whether or not the following
character would otherwise be interpreted as a metacharacter, so it is
always safe to precede a non-alphanumeric with backslash to specify
that it stands for itself. In particular, if you want to match a back‐
slash, you write \\.
In a UTF mode, only ASCII numbers and letters have any special meaning
after a backslash. All other characters (in particular, those whose
codepoints are greater than 127) are treated as literals.
If a pattern is compiled with the PCRE_EXTENDED option, most white
space in the pattern (other than in a character class), and characters
between a # outside a character class and the next newline, inclusive,
are ignored. An escaping backslash can be used to include a white space
or # character as part of the pattern.
If you want to remove the special meaning from a sequence of charac‐
ters, you can do so by putting them between \Q and \E. This is differ‐
ent from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE, whereas in Perl, $ and @ cause variable interpola‐
tion. Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes. An isolated \E that is not preceded by \Q is ignored. If \Q
is not followed by \E later in the pattern, the literal interpretation
continues to the end of the pattern (that is, \E is assumed at the
end). If the isolated \Q is inside a character class, this causes an
error, because the character class is not terminated.
Non-printing characters
A second use of backslash provides a way of encoding non-printing char‐
acters in patterns in a visible manner. There is no restriction on the
appearance of non-printing characters, apart from the binary zero that
terminates a pattern, but when a pattern is being prepared by text
editing, it is often easier to use one of the following escape
sequences than the binary character it represents. In an ASCII or Uni‐
code environment, these escapes are as follows:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any ASCII character
\e escape (hex 1B)
\f form feed (hex 0C)
\n linefeed (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\0dd character with octal code 0dd
\ddd character with octal code ddd, or back reference
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh.. (non-JavaScript mode)
\uhhhh character with hex code hhhh (JavaScript mode only)
The precise effect of \cx on ASCII characters is as follows: if x is a
lower case letter, it is converted to upper case. Then bit 6 of the
character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
(A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c
has a value greater than 127, a compile-time error occurs. This locks
out non-ASCII characters in all modes.
When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t gener‐
ate the appropriate EBCDIC code values. The \c escape is processed as
specified for Perl in the perlebcdic document. The only characters that
are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?.
Any other character provokes a compile-time error. The sequence \@
encodes character code 0; the letters (in either case) encode charac‐
ters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31
(hex 1B to hex 1F), and \? becomes either 255 (hex FF) or 95 (hex 5F).
Thus, apart from \?, these escapes generate the same character code
values as they do in an ASCII environment, though the meanings of the
values mostly differ. For example, \G always generates code value 7,
which is BEL in ASCII but DEL in EBCDIC.
The sequence \? generates DEL (127, hex 7F) in an ASCII environment,
but because 127 is not a control character in EBCDIC, Perl makes it
generate the APC character. Unfortunately, there are several variants
of EBCDIC. In most of them the APC character has the value 255 (hex
FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If
certain other characters have POSIX-BC values, PCRE makes \? generate
95; otherwise it generates 255.
After \0 up to two further octal digits are read. If there are fewer
than two digits, just those that are present are used. Thus the
sequence \0\x\015 specifies two binary zeros followed by a CR character
(code value 13). Make sure you supply two digits after the initial zero
if the pattern character that follows is itself an octal digit.
The escape \o must be followed by a sequence of octal digits, enclosed
in braces. An error occurs if this is not the case. This escape is a
recent addition to Perl; it provides way of specifying character code
points as octal numbers greater than 0777, and it also allows octal
numbers and back references to be unambiguously specified.
For greater clarity and unambiguity, it is best to avoid following \ by
a digit greater than zero. Instead, use \o{} or \x{} to specify charac‐
ter numbers, and \g{} to specify back references. The following para‐
graphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is compli‐
cated, and Perl has changed in recent releases, causing PCRE also to
change. Outside a character class, PCRE reads the digit and any follow‐
ing digits as a decimal number. If the number is less than 8, or if
there have been at least that many previous capturing left parentheses
in the expression, the entire sequence is taken as a back reference. A
description of how this works is given later, following the discussion
of parenthesized subpatterns.
Inside a character class, or if the decimal number following \ is
greater than 7 and there have not been that many capturing subpatterns,
PCRE handles \8 and \9 as the literal characters "8" and "9", and oth‐
erwise re-reads up to three octal digits following the backslash, using
them to generate a data character. Any subsequent digits stand for
themselves. For example:
\040 is another way of writing an ASCII space
\40 is the same, provided there are fewer than 40
previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of
writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 might be a back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the value 255 (decimal)
\81 is either a back reference, or the two
characters "8" and "1"
Note that octal values of 100 or greater that are specified using this
syntax must not be introduced by a leading zero, because no more than
three octal digits are ever read.
By default, after \x that is not followed by {, from zero to two hexa‐
decimal digits are read (letters can be in upper or lower case). Any
number of hexadecimal digits may appear between \x{ and }. If a charac‐
ter other than a hexadecimal digit appears between \x{ and }, or if
there is no terminating }, an error occurs.
If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
is as just described only when it is followed by two hexadecimal dig‐
its. Otherwise, it matches a literal "x" character. In JavaScript
mode, support for code points greater than 256 is provided by \u, which
must be followed by four hexadecimal digits; otherwise it matches a
literal "u" character.
Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x (or by \u in JavaScript mode). There is no differ‐
ence in the way they are handled. For example, \xdc is exactly the same
as \x{dc} (or \u00dc in JavaScript mode).
Constraints on character values
Characters that are specified using octal or hexadecimal numbers are
limited to certain values, as follows:
8-bit non-UTF mode less than 0x100
8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
16-bit non-UTF mode less than 0x10000
16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
32-bit non-UTF mode less than 0x100000000
32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
called "surrogate" codepoints), and 0xffef.
Escape sequences in character classes
All the sequences that define a single character value can be used both
inside and outside character classes. In addition, inside a character
class, \b is interpreted as the backspace character (hex 08).
\N is not allowed in a character class. \B, \R, and \X are not special
inside a character class. Like other unrecognized escape sequences,
they are treated as the literal characters "B", "R", and "X" by
default, but cause an error if the PCRE_EXTRA option is set. Outside a
character class, these sequences have different meanings.
Unsupported escape sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by its string
handler and used to modify the case of following characters. By
default, PCRE does not support these escape sequences. However, if the
PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
\u can be used to define a character by code point, as described in the
previous section.
Absolute and relative back references
The sequence \g followed by an unsigned or a negative number, option‐
ally enclosed in braces, is an absolute or relative back reference. A
named back reference can be coded as \g{name}. Back references are dis‐
cussed later, following the discussion of parenthesized subpatterns.
Absolute and relative subroutine calls
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
an alternative syntax for referencing a subpattern as a "subroutine".
Details are discussed later. Note that \g{...} (Perl syntax) and
\g<...> (Oniguruma syntax) are not synonymous. The former is a back
reference; the latter is a subroutine call.
Generic character types
Another use of backslash is for specifying generic character types:
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space character
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
\V any character that is not a vertical white space character
\w any "word" character
\W any "non-word" character
There is also the single sequence \N, which matches a non-newline char‐
acter. This is the same as the "." metacharacter when PCRE_DOTALL is
not set. Perl also uses \N to match characters by name; PCRE does not
support this.
Each pair of lower and upper case escape sequences partitions the com‐
plete set of characters into two disjoint sets. Any given character
matches one, and only one, of each pair. The sequences can appear both
inside and outside character classes. They each match one character of
the appropriate type. If the current matching point is at the end of
the subject string, all of them fail, because there is no character to
match.
For compatibility with Perl, \s did not used to match the VT character
(code 11), which made it different from the the POSIX "space" class.
However, Perl added VT at release 5.18, and PCRE followed suit at
release 8.34. The default \s characters are now HT (9), LF (10), VT
(11), FF (12), CR (13), and space (32), which are defined as white
space in the "C" locale. This list may vary if locale-specific matching
is taking place. For example, in some locales the "non-breaking space"
character (\xA0) is recognized as white space, and in others the VT
character is not.
A "word" character is an underscore or any character that is a letter
or digit. By default, the definition of letters and digits is con‐
trolled by PCRE's low-valued character tables, and may vary if locale-
specific matching is taking place (see "Locale support" in the pcreapi
page). For example, in a French locale such as "fr_FR" in Unix-like
systems, or "french" in Windows, some character codes greater than 127
are used for accented letters, and these are then matched by \w. The
use of locales with Unicode is discouraged.
By default, characters whose code points are greater than 127 never
match \d, \s, or \w, and always match \D, \S, and \W, although this may
vary for characters in the range 128-255 when locale-specific matching
is happening. These escape sequences retain their original meanings
from before Unicode support was available, mainly for efficiency rea‐
sons. If PCRE is compiled with Unicode property support, and the
PCRE_UCP option is set, the behaviour is changed so that Unicode prop‐
erties are used to determine character types, as follows:
\d any character that matches \p{Nd} (decimal digit)
\s any character that matches \p{Z} or \h or \v
\w any character that matches \p{L} or \p{N}, plus underscore
The upper case escapes match the inverse sets of characters. Note that
\d matches only decimal digits, whereas \w matches any Unicode digit,
as well as any Unicode letter, and underscore. Note also that PCRE_UCP
affects \b, and \B because they are defined in terms of \w and \W.
Matching these sequences is noticeably slower when PCRE_UCP is set.
The sequences \h, \H, \v, and \V are features that were added to Perl
at release 5.10. In contrast to the other sequences, which match only
ASCII characters by default, these always match certain high-valued
code points, whether or not PCRE_UCP is set. The horizontal space char‐
acters are:
U+0009 Horizontal tab (HT)
U+0020 Space
U+00A0 Non-break space
U+1680 Ogham space mark
U+180E Mongolian vowel separator
U+2000 En quad
U+2001 Em quad
U+2002 En space
U+2003 Em space
U+2004 Three-per-em space
U+2005 Four-per-em space
U+2006 Six-per-em space
U+2007 Figure space
U+2008 Punctuation space
U+2009 Thin space
U+200A Hair space
U+202F Narrow no-break space
U+205F Medium mathematical space
U+3000 Ideographic space
The vertical space characters are:
U+000A Linefeed (LF)
U+000B Vertical tab (VT)
U+000C Form feed (FF)
U+000D Carriage return (CR)
U+0085 Next line (NEL)
U+2028 Line separator
U+2029 Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
256 are relevant.
Newline sequences
Outside a character class, by default, the escape sequence \R matches
any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which are given
below. This particular group matches either the two-character sequence
CR followed by LF, or one of the single characters LF (linefeed,
U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car‐
riage return, U+000D), or NEL (next line, U+0085). The two-character
sequence is treated as a single unit that cannot be split.
In other modes, two additional characters whose codepoints are greater
than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa‐
rator, U+2029). Unicode character property support is not needed for
these characters to be recognized.
It is possible to restrict \R to match only CR, LF, or CRLF (instead of
the complete set of Unicode line endings) by setting the option
PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
(BSR is an abbrevation for "backslash R".) This can be made the default
when PCRE is built; if this is the case, the other behaviour can be
requested via the PCRE_BSR_UNICODE option. It is also possible to
specify these settings by starting a pattern string with one of the
following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence
These override the default and the options given to the compiling func‐
tion, but they can themselves be overridden by options given to a
matching function. Note that these special settings, which are not
Perl-compatible, are recognized only at the very start of a pattern,
and that they must be in upper case. If more than one of them is
present, the last one is used. They can be combined with a change of
newline convention; for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF)
or (*UCP) special sequences. Inside a character class, \R is treated as
an unrecognized escape sequence, and so matches the letter "R" by
default, but causes an error if PCRE_EXTRA is set.
Unicode character properties
When PCRE is built with Unicode character property support, three addi‐
tional escape sequences that match characters with specific properties
are available. When in 8-bit non-UTF-8 mode, these sequences are of
course limited to testing characters whose codepoints are less than
256, but they do work in this mode. The extra escape sequences are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\X a Unicode extended grapheme cluster
The property names represented by xx above are limited to the Unicode
script names, the general category properties, "Any", which matches any
character (including newline), and some special PCRE properties
(described in the next section). Other Perl properties such as "InMu‐
sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
does not match any characters, so always causes a match failure.
Sets of Unicode characters are defined as belonging to certain scripts.
A character from one of these sets can be matched using a script name.
For example:
\p{Greek}
\P{Han}
Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali,
Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car‐
ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei‐
form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero‐
glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha,
Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,
Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip‐
tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin‐
ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic,
Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive,
Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean,
New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian,
Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya,
Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician,
Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha‐
vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac,
Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu,
Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi,
Yi.
Each character has exactly one Unicode general category property, spec‐
ified by a two-letter abbreviation. For compatibility with Perl, nega‐
tion can be specified by including a circumflex between the opening
brace and the property name. For example, \p{^Lu} is the same as
\P{Lu}.
If only one letter is specified with \p or \P, it includes all the gen‐
eral category properties that start with that letter. In this case, in
the absence of negation, the curly brackets in the escape sequence are
optional; these two examples have the same effect:
\p{L}
\pL
The following general category property codes are supported:
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
The special property L& is also supported: it matches a character that
has the Lu, Ll, or Lt property, in other words, a letter that is not
classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range
U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
so cannot be tested by PCRE, unless UTF validity checking has been
turned off (see the discussion of PCRE_NO_UTF8_CHECK,
PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl
does not support the Cs property.
The long synonyms for property names that Perl supports (such as
\p{Letter}) are not supported by PCRE, nor is it permitted to prefix
any of these properties with "Is".
No character that is in the Unicode table has the Cn (unassigned) prop‐
erty. Instead, this property is assumed for any code point that is not
in the Unicode table.
Specifying caseless matching does not affect these escape sequences.
For example, \p{Lu} always matches only upper case letters. This is
different from the behaviour of current versions of Perl.
Matching characters by Unicode property is not fast, because PCRE has
to do a multistage table lookup in order to find a character's prop‐
erty. That is why the traditional escape sequences such as \d and \w do
not use Unicode properties in PCRE by default, though you can make them
do so by setting the PCRE_UCP option or by starting the pattern with
(*UCP).
Extended grapheme clusters
The \X escape matches any number of Unicode characters that form an
"extended grapheme cluster", and treats the sequence as an atomic group
(see below). Up to and including release 8.31, PCRE matched an ear‐
lier, simpler definition that was equivalent to
(?>\PM\pM*)
That is, it matched a character without the "mark" property, followed
by zero or more characters with the "mark" property. Characters with
the "mark" property are typically non-spacing accents that affect the
preceding character.
This simple definition was extended in Unicode to include more compli‐
cated kinds of composite character by giving each character a grapheme
breaking property, and creating rules that use these properties to
define the boundaries of extended grapheme clusters. In releases of
PCRE later than 8.31, \X matches one of these clusters.
\X always matches at least one character. Then it decides whether to
add additional characters according to the following rules for ending a
cluster:
1. End at the end of the subject string.
2. Do not end between CR and LF; otherwise end after any control char‐
acter.
3. Do not break Hangul (a Korean script) syllable sequences. Hangul
characters are of five types: L, V, T, LV, and LVT. An L character may
be followed by an L, V, LV, or LVT character; an LV or V character may
be followed by a V or T character; an LVT or T character may be follwed
only by a T character.
4. Do not end before extending characters or spacing marks. Characters
with the "mark" property always have the "extend" grapheme breaking
property.
5. Do not end after prepend characters.
6. Otherwise, end the cluster.
PCRE's additional properties
As well as the standard Unicode properties described above, PCRE sup‐
ports four more that make it possible to convert traditional escape
sequences such as \w and \s to use Unicode properties. PCRE uses these
non-standard, non-Perl properties internally when PCRE_UCP is set. How‐
ever, they may also be used explicitly. These properties are:
Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character
Xan matches characters that have either the L (letter) or the N (num‐
ber) property. Xps matches the characters tab, linefeed, vertical tab,
form feed, or carriage return, and any other character that has the Z
(separator) property. Xsp is the same as Xps; it used to exclude ver‐
tical tab, for Perl compatibility, but Perl changed, and so PCRE fol‐
lowed at release 8.34. Xwd matches the same characters as Xan, plus
underscore.
There is another non-standard property, Xuc, which matches any charac‐
ter that can be represented by a Universal Character Name in C++ and
other programming languages. These are the characters $, @, ` (grave
accent), and all characters with Unicode code points greater than or
equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that
most base (ASCII) characters are excluded. (Universal Character Names
are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit.
Note that the Xuc property does not match these sequences but the char‐
acters that they represent.)
Resetting the match start
The escape sequence \K causes any previously matched characters not to
be included in the final matched sequence. For example, the pattern:
foo\Kbar
matches "foobar", but reports that it has matched "bar". This feature
is similar to a lookbehind assertion (described below). However, in
this case, the part of the subject before the real match does not have
to be of fixed length, as lookbehind assertions do. The use of \K does
not interfere with the setting of captured substrings. For example,
when the pattern
(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
Perl documents that the use of \K within assertions is "not well
defined". In PCRE, \K is acted upon when it occurs inside positive
assertions, but is ignored in negative assertions. Note that when a
pattern such as (?=ab\K) matches, the reported start of the match can
be greater than the end of the match.
Simple assertions
The final use of backslash is for certain simple assertions. An asser‐
tion specifies a condition that has to be met at a particular point in
a match, without consuming any characters from the subject string. The
use of subpatterns for more complicated assertions is described below.
The backslashed assertions are:
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
\G matches at the first matching position in the subject
Inside a character class, \b has a different meaning; it matches the
backspace character. If any other of these assertions appears in a
character class, by default it matches the corresponding literal char‐
acter (for example, \B matches the letter B). However, if the
PCRE_EXTRA option is set, an "invalid escape sequence" error is gener‐
ated instead.
A word boundary is a position in the subject string where the current
character and the previous character do not both match \w or \W (i.e.
one matches \w and the other matches \W), or the start or end of the
string if the first or last character matches \w, respectively. In a
UTF mode, the meanings of \w and \W can be changed by setting the
PCRE_UCP option. When this is done, it also affects \b and \B. Neither
PCRE nor Perl has a separate "start of word" or "end of word" metase‐
quence. However, whatever follows \b normally determines which it is.
For example, the fragment \ba matches "a" at the start of a word.
The \A, \Z, and \z assertions differ from the traditional circumflex
and dollar (described in the next section) in that they only ever match
at the very start and end of the subject string, whatever options are
set. Thus, they are independent of multiline mode. These three asser‐
tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
affect only the behaviour of the circumflex and dollar metacharacters.
However, if the startoffset argument of pcre_exec() is non-zero, indi‐
cating that matching is to start at a point other than the beginning of
the subject, \A can never match. The difference between \Z and \z is
that \Z matches before a newline at the end of the string as well as at
the very end, whereas \z matches only at the end.
The \G assertion is true only when the current matching position is at
the start point of the match, as specified by the startoffset argument
of pcre_exec(). It differs from \A when the value of startoffset is
non-zero. By calling pcre_exec() multiple times with appropriate argu‐
ments, you can mimic Perl's /g option, and it is in this kind of imple‐
mentation where \G can be useful.
Note, however, that PCRE's interpretation of \G, as the start of the
current match, is subtly different from Perl's, which defines it as the
end of the previous match. In Perl, these can be different when the
previously matched string was empty. Because PCRE does just one match
at a time, it cannot reproduce this behaviour.
If all the alternatives of a pattern begin with \G, the expression is
anchored to the starting match position, and the "anchored" flag is set
in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
The circumflex and dollar metacharacters are zero-width assertions.
That is, they test for a particular condition being true without con‐
suming any characters from the subject string.
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching
point is at the start of the subject string. If the startoffset argu‐
ment of pcre_exec() is non-zero, circumflex can never match if the
PCRE_MULTILINE option is unset. Inside a character class, circumflex
has an entirely different meaning (see below).
Circumflex need not be the first character of the pattern if a number
of alternatives are involved, but it should be the first thing in each
alternative in which it appears if the pattern is ever to match that
branch. If all possible alternatives start with a circumflex, that is,
if the pattern is constrained to match only at the start of the sub‐
ject, it is said to be an "anchored" pattern. (There are also other
constructs that can cause a pattern to be anchored.)
The dollar character is an assertion that is true only if the current
matching point is at the end of the subject string, or immediately
before a newline at the end of the string (by default). Note, however,
that it does not actually match the newline. Dollar need not be the
last character of the pattern if a number of alternatives are involved,
but it should be the last item in any branch in which it appears. Dol‐
lar has no special meaning in a character class.
The meaning of dollar can be changed so that it matches only at the
very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
compile time. This does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, a circumflex
matches immediately after internal newlines as well as at the start of
the subject string. It does not match after a newline that ends the
string. A dollar matches before any newlines in the string, as well as
at the very end, when PCRE_MULTILINE is set. When newline is specified
as the two-character sequence CRLF, isolated CR and LF characters do
not indicate newlines.
For example, the pattern /^abc$/ matches the subject string "def\nabc"
(where \n represents a newline) in multiline mode, but not otherwise.
Consequently, patterns that are anchored in single line mode because
all branches start with ^ are not anchored in multiline mode, and a
match for circumflex is possible when the startoffset argument of
pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.
Note that the sequences \A, \Z, and \z can be used to match the start
and end of the subject in both modes, and if all branches of a pattern
start with \A it is always anchored, whether or not PCRE_MULTILINE is
set.
FULL STOP (PERIOD, DOT) AND \N
Outside a character class, a dot in the pattern matches any one charac‐
ter in the subject string except (by default) a character that signi‐
fies the end of a line.
When a line ending is defined as a single character, dot never matches
that character; when the two-character sequence CRLF is used, dot does
not match CR if it is immediately followed by LF, but otherwise it
matches all characters (including isolated CRs and LFs). When any Uni‐
code line endings are being recognized, dot does not match CR or LF or
any of the other line ending characters.
The behaviour of dot with regard to newlines can be changed. If the
PCRE_DOTALL option is set, a dot matches any one character, without
exception. If the two-character sequence CRLF is present in the subject
string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circum‐
flex and dollar, the only relationship being that they both involve
newlines. Dot has no special meaning in a character class.
The escape sequence \N behaves like a dot, except that it is not
affected by the PCRE_DOTALL option. In other words, it matches any
character except one that signifies the end of a line. Perl also uses
\N to match characters by name; PCRE does not support this.
MATCHING A SINGLE DATA UNIT
Outside a character class, the escape sequence \C matches any one data
unit, whether or not a UTF mode is set. In the 8-bit library, one data
unit is one byte; in the 16-bit library it is a 16-bit unit; in the
32-bit library it is a 32-bit unit. Unlike a dot, \C always matches
line-ending characters. The feature is provided in Perl in order to
match individual bytes in UTF-8 mode, but it is unclear how it can use‐
fully be used. Because \C breaks up characters into individual data
units, matching one unit with \C in a UTF mode means that the rest of
the string may start with a malformed UTF character. This has undefined
results, because PCRE assumes that it is dealing with valid UTF strings
(and by default it checks this at the start of processing unless the
PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option
is used).
PCRE does not allow \C to appear in lookbehind assertions (described
below) in a UTF mode, because this would make it impossible to calcu‐
late the length of the lookbehind.
In general, the \C escape sequence is best avoided. However, one way of
using it that avoids the problem of malformed UTF characters is to use
a lookahead to check the length of the next character, as in this pat‐
tern, which could be used with a UTF-8 string (ignore white space and
line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
A group that starts with (?| resets the capturing parentheses numbers
in each alternative (see "Duplicate Subpattern Numbers" below). The
assertions at the start of each branch check the next UTF-8 character
for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
character's individual bytes are then captured by the appropriate num‐
ber of groups.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe‐
cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
a lone closing square bracket causes a compile-time error. If a closing
square bracket is required as a member of the class, it should be the
first data character in the class (after an initial circumflex, if
present) or escaped with a backslash.
A character class matches a single character in the subject. In a UTF
mode, the character may be more than one data unit long. A matched
character must be in the set of characters defined by the class, unless
the first character in the class definition is a circumflex, in which
case the subject character must not be in the set defined by the class.
If a circumflex is actually required as a member of the class, ensure
it is not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lower case vowel,
while [^aeiou] matches any character that is not a lower case vowel.
Note that a circumflex is just a convenient notation for specifying the
characters that are in the class by enumerating those that are not. A
class that starts with a circumflex is not an assertion; it still con‐
sumes a character from the subject string, and therefore it fails if
the current pointer is at the end of the string.
In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255
(0xffff) can be included in a class as a literal string of data units,
or by using the \x{ escaping mechanism.
When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, so for example, a caseless
[aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would. In a UTF mode, PCRE always
understands the concept of case for characters whose values are less
than 128, so caseless matching is always possible. For characters with
higher values, the concept of case is supported if PCRE is compiled
with Unicode property support, but not otherwise. If you want to use
caseless matching in a UTF mode for characters 128 and above, you must
ensure that PCRE is compiled with Unicode property support as well as
with UTF support.
Characters that might indicate line breaks are never treated in any
special way when matching character classes, whatever line-ending
sequence is in use, and whatever setting of the PCRE_DOTALL and
PCRE_MULTILINE options is used. A class such as [^a] always matches one
of these characters.
The minus (hyphen) character can be used to specify a range of charac‐
ters in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class, or immediately after a range. For
example, [b-d-z] matches letters in the range b to d, a hyphen charac‐
ter, or z.
It is not possible to have the literal character "]" as the end charac‐
ter of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is inter‐
preted as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end
a range.
An error is generated if a POSIX character class (see below) or an
escape sequence other than one that defines a single character appears
at a point where a range ending character is expected. For example,
[z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.
Ranges operate in the collating sequence of character values. They can
also be used for characters specified numerically, for example
[\000-\037]. Ranges can include any characters that are valid for the
current mode.
If a range that includes letters is used when caseless matching is set,
it matches the letters in either case. For example, [W-c] is equivalent
to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
character tables for a French locale are in use, [\xc8-\xcb] matches
accented E characters in both cases. In UTF modes, PCRE supports the
concept of case for characters with values greater than 128 only when
it is compiled with Unicode property support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
\w, and \W may appear in a character class, and add the characters that
they match to the class. For example, [\dABCDEF] matches any hexadeci‐
mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
\d, \s, \w and their upper case partners, just as it does when they
appear outside a character class, as described in the section entitled
"Generic character types" above. The escape sequence \b has a different
meaning inside a character class; it matches the backspace character.
The sequences \B, \N, \R, and \X are not special inside a character
class. Like any other unrecognized escape sequences, they are treated
as the literal characters "B", "N", "R", and "X" by default, but cause
an error if the PCRE_EXTRA option is set.
A circumflex can conveniently be used with the upper case character
types to specify a more restricted set of characters than the matching
lower case type. For example, the class [^\W_] matches any letter or
digit, but not underscore, whereas [\w] includes underscore. A positive
character class should be read as "something OR something OR ..." and a
negative class as "NOT something AND NOT something AND NOT ...".
The only metacharacters that are recognized in character classes are
backslash, hyphen (only where it can be interpreted as specifying a
range), circumflex (only at the start), opening square bracket (only
when it can be interpreted as introducing a POSIX class name, or for a
special compatibility feature - see the next two sections), and the
terminating closing square bracket. However, escaping other non-
alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also
supports this notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The supported class
names are:
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits and space
space white space (the same as \s from PCRE 8.34)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The default "space" characters are HT (9), LF (10), VT (11), FF (12),
CR (13), and space (32). If locale-specific matching is taking place,
the list of space characters may be different; there may be fewer or
more of them. "Space" used to be different to \s, which did not include
VT, for Perl compatibility. However, Perl changed at release 5.18, and
PCRE followed at release 8.34. "Space" and \s now match the same set
of characters.
The name "word" is a Perl extension, and "blank" is a GNU extension
from Perl 5.8. Another Perl extension is negation, which is indicated
by a ^ character after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
these are not supported, and an error is given if they are encountered.
By default, characters with values greater than 128 do not match any of
the POSIX character classes. However, if the PCRE_UCP option is passed
to pcre_compile(), some of the classes are changed so that Unicode
character properties are used. This is achieved by replacing certain
POSIX classes by other sequences, as follows:
[:alnum:] becomes \p{Xan}
[:alpha:] becomes \p{L}
[:blank:] becomes \h
[:digit:] becomes \p{Nd}
[:lower:] becomes \p{Ll}
[:space:] becomes \p{Xps}
[:upper:] becomes \p{Lu}
[:word:] becomes \p{Xwd}
Negated versions, such as [:^alpha:] use \P instead of \p. Three other
POSIX classes are handled specially in UCP mode:
[:graph:] This matches characters that have glyphs that mark the page
when printed. In Unicode property terms, it matches all char‐
acters with the L, M, N, P, S, or Cf properties, except for:
U+061C Arabic Letter Mark
U+180E Mongolian Vowel Separator
U+2066 - U+2069 Various "isolate"s
[:print:] This matches the same characters as [:graph:] plus space
characters that are not controls, that is, characters with
the Zs property.
[:punct:] This matches all characters that have the Unicode P (punctua‐
tion) property, plus those characters whose code points are
less than 128 that have the S (Symbol) property.
The other POSIX classes are unchanged, and match only characters with
code points less than 128.
COMPATIBILITY FEATURE FOR WORD BOUNDARIES
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the
ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word"
and "end of word". PCRE treats these items as follows:
[[:<:]] is converted to \b(?=\w)
[[:>:]] is converted to \b(?<=\w)
Only these exact character sequences are recognized. A sequence such as
[a[:<:]b] provokes error for an unrecognized POSIX class name. This
support is not compatible with Perl. It is provided to help migrations
from other environments, and is best not used in any new patterns. Note
that \b matches at the start and the end of a word (see "Simple asser‐
tions" above), and in a Perl-style pattern the preceding or following
character normally shows which is wanted, without the need for the
assertions that are used above in order to give exactly the POSIX be‐
haviour.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For
example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of alternatives may
appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from left
to right, and the first one that succeeds is used. If the alternatives
are within a subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the subpattern.
INTERNAL OPTION SETTING
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options (which are Perl-compatible) can be changed from
within the pattern by a sequence of Perl option letters enclosed
between "(?" and ")". The option letters are
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possi‐
ble to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE_CASE‐
LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
can be changed in the same way as the Perl-compatible options by using
the characters J, U and X respectively.
When one of these option changes occurs at top level (that is, not
inside subpattern parentheses), the change applies to the remainder of
the pattern that follows. If the change is placed right at the start of
a pattern, PCRE extracts it into the global options (and it will there‐
fore show up in data extracted by the pcre_fullinfo() function).
An option change within a subpattern (see below for a description of
subpatterns) affects only that part of the subpattern that follows it,
so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
used). By this means, options can be made to have different settings
in different parts of the pattern. Any changes made in one alternative
do carry on into subsequent branches within the same subpattern. For
example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the
first branch is abandoned before the option setting. This is because
the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.
Note: There are other PCRE-specific options that can be set by the
application when the compiling or matching functions are called. In
some cases the pattern can contain special leading sequences such as
(*CRLF) to override what the application has set or what has been
defaulted. Details are given in the section entitled "Newline
sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and
(*UCP) leading sequences that can be used to set UTF and Unicode prop‐
erty modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16,
PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF) sequence
is a generic version that can be used with any of the libraries. How‐
ever, the application can set the PCRE_NEVER_UTF option, which locks
out the use of the (*UTF) sequences.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Turning part of a pattern into a subpattern does two things:
1. It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|)
matches "cataract", "caterpillar", or "cat". Without the parentheses,
it would match "cataract", "erpillar" or an empty string.
2. It sets up the subpattern as a capturing subpattern. This means
that, when the whole pattern matches, that portion of the subject
string that matched the subpattern is passed back to the caller via the
ovector argument of the matching function. (This applies only to the
traditional matching functions; the DFA matching functions do not sup‐
port capturing.)
Opening parentheses are counted from left to right (starting from 1) to
obtain numbers for the capturing subpatterns. For example, if the
string "the red king" is matched against the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are num‐
bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by a question mark and a colon, the subpattern does not do any captur‐
ing, and is not counted when computing the number of any subsequent
capturing subpatterns. For example, if the string "the white queen" is
matched against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are numbered
1 and 2. The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are
tried from left to right, and options are not reset until the end of
the subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern
uses the same numbers for its capturing parentheses. Such a subpattern
starts with (?| and is itself a non-capturing subpattern. For example,
consider this pattern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both sets of cap‐
turing parentheses are numbered one. Thus, when the pattern matches,
you can look at captured substring number one, whichever alternative
matched. This construct is useful when you want to capture part, but
not all, of one of a number of alternatives. Inside a (?| group, paren‐
theses are numbered as usual, but the number is reset at the start of
each branch. The numbers of any capturing parentheses that follow the
subpattern start after the highest number used in any branch. The fol‐
lowing example is taken from the Perl documentation. The numbers under‐
neath show in which buffer the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A back reference to a numbered subpattern uses the most recent value
that is set for that number by any subpattern. The following pattern
matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern always refers
to the first one in the pattern with the given number. The following
pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
If a condition test for a subpattern's having matched refers to a non-
unique number, the test is true if any of the subpatterns of that num‐
ber have matched.
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expres‐
sions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE supports the naming of sub‐
patterns. This feature was not added to Perl until release 5.10. Python
had the feature earlier, and PCRE introduced it at release 4.0, using
the Python syntax. PCRE now supports both the Perl and the Python syn‐
tax. Perl allows identically numbered subpatterns to have different
names, but PCRE does not.
In PCRE, a subpattern can be named in one of three ways: (?<name>...)
or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
to capturing parentheses from other parts of the pattern, such as back
references, recursion, and conditions, can be made by name as well as
by number.
Names consist of up to 32 alphanumeric characters and underscores, but
must start with a non-digit. Named capturing parentheses are still
allocated numbers as well as names, exactly as if the names were not
present. The PCRE API provides function calls for extracting the name-
to-number translation table from a compiled pattern. There is also a
convenience function for extracting a captured substring by name.
By default, a name must be unique within a pattern, but it is possible
to relax this constraint by setting the PCRE_DUPNAMES option at compile
time. (Duplicate names are also always permitted for subpatterns with
the same number, set up as described in the previous section.) Dupli‐
cate names can be useful for patterns where only one instance of the
named parentheses can match. Suppose you want to match the name of a
weekday, either as a 3-letter abbreviation or as the full name, and in
both cases you want to extract the abbreviation. This pattern (ignoring
the line breaks) does the job:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
There are five capturing substrings, but only one is ever set after a
match. (An alternative way of solving this problem is to use a "branch
reset" subpattern, as described in the previous section.)
The convenience function for extracting the data by name returns the
substring for the first (and in this example, the only) subpattern of
that name that matched. This saves searching to find which numbered
subpattern it was.
If you make a back reference to a non-unique named subpattern from
elsewhere in the pattern, the subpatterns to which the name refers are
checked in the order in which they appear in the overall pattern. The
first one that is set is used for the reference. For example, this pat‐
tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":
(?:(?<n>foo)|(?<n>bar))\k<n>
If you make a subroutine call to a non-unique named subpattern, the one
that corresponds to the first occurrence of the name is used. In the
absence of duplicate numbers (see the previous section) this is the one
with the lowest number.
If you use a named reference in a condition test (see the section about
conditions below), either to check whether a subpattern has matched, or
to check for recursion, all subpatterns with the same name are tested.
If the condition is true for any one of them, the overall condition is
true. This is the same behaviour as testing by number. For further
details of the interfaces for handling named subpatterns, see the
pcreapi documentation.
Warning: You cannot use different names to distinguish between two sub‐
patterns with the same number because PCRE uses only the numbers when
matching. For this reason, an error is given at compile time if differ‐
ent names are given to subpatterns with the same number. However, you
can always give the same name to subpatterns with the same number, even
when PCRE_DUPNAMES is not set.
REPETITION
Repetition is specified by quantifiers, which can follow any of the
following items:
a literal data character
the dot metacharacter
the \C escape sequence
the \X escape sequence
the \R escape sequence
an escape such as \d or \pL that matches a single character
a character class
a back reference (see next section)
a parenthesized subpattern (including assertions)
a subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum num‐
ber of permitted matches, by giving the two numbers in curly brackets
(braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example:
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
special character. If the second number is omitted, but the comma is
present, there is no upper limit; if the second number and the comma
are both omitted, the quantifier specifies an exact number of required
matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more, while
\d{8}
matches exactly 8 digits. An opening curly bracket that appears in a
position where a quantifier is not allowed, or one that does not match
the syntax of a quantifier, is taken as a literal character. For exam‐
ple, {,6} is not a quantifier, but a literal string of four characters.
In UTF modes, quantifiers apply to characters rather than to individual
data units. Thus, for example, \x{100}{2} matches two characters, each
of which is represented by a two-byte sequence in a UTF-8 string. Simi‐
larly, \X{3} matches three Unicode extended grapheme clusters, each of
which may be several data units long (and they may be of different
lengths).
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present. This may be use‐
ful for subpatterns that are referenced as subroutines from elsewhere
in the pattern (but see also the section entitled "Defining subpatterns
for use by reference only" below). Items other than subpatterns that
have a {0} quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-charac‐
ter abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern
that can match no characters with a quantifier that has no upper limit,
for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
for such patterns. However, because there are cases where this can be
useful, such patterns are now accepted, but if any repetition of the
subpattern does in fact match no characters, the loop is forcibly bro‐
ken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear between /* and */ and within the comment, individual * and /
characters may appear. An attempt to match C comments by applying the
pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, because it matches the entire string owing to the greediness of
the .* item.
However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible, so
the pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the
only way the rest of the pattern matches.
If the PCRE_UNGREEDY option is set (an option that is not available in
Perl), the quantifiers are not greedy by default, but individual ones
can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
When a parenthesized subpattern is quantified with a minimum repeat
count that is greater than 1 or with a limited maximum, more memory is
required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv‐
alent to Perl's /s) is set, thus allowing the dot to match newlines,
the pattern is implicitly anchored, because whatever follows will be
tried against every character position in the subject string, so there
is no point in retrying the overall match at any position after the
first. PCRE normally treats such a pattern as though it were preceded
by \A.
In cases where it is known that the subject string contains no new‐
lines, it is worth setting PCRE_DOTALL in order to obtain this opti‐
mization, or alternatively using ^ to indicate anchoring explicitly.
However, there are some cases where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a back
reference elsewhere in the pattern, a match at the start may fail where
a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth charac‐
ter. For this reason, such a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the lead‐
ing .* is inside an atomic group. Once again, a match at the start may
fail where a later one succeeds. Consider this pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the backtracking con‐
trol verbs (*PRUNE) and (*SKIP) also disable this optimization.
When a capturing subpattern is repeated, the value captured is the sub‐
string that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns,
the corresponding captured values may have been set in previous itera‐
tions. For example, after
/(a|(b))+/
matches "aba" the value of the second captured substring is "b".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item
to be re-evaluated to see if a different number of repeats allows the
rest of the pattern to match. Sometimes it is useful to prevent this,
either to change the nature of the match, or to cause it fail earlier
than it otherwise might, when the author of the pattern knows there is
no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the subject
line
123456bar
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the
\d+ item, and then with 4, and so on, before ultimately failing.
"Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
the means for specifying that once a subpattern has matched, it is not
to be re-evaluated in this way.
If we use atomic grouping for the previous example, the matcher gives
up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern it con‐
tains once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases
such as the above example can be thought of as a maximizing repeat that
must swallow everything it can. So, while both \d+ and \d+? are pre‐
pared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of
digits.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an
atomic group is just a single repeated item, as in the example above, a
simpler notation, called a "possessive quantifier" can be used. This
consists of an additional + character following a quantifier. Using
this notation, the previous example can be rewritten as
\d++foo
Note that a possessive quantifier can be used with an entire group, for
example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of the
PCRE_UNGREEDY option is ignored. They are a convenient notation for the
simpler forms of atomic group. However, there is no difference in the
meaning of a possessive quantifier and the equivalent atomic group,
though there may be a performance difference; possessive quantifiers
should be slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8 syn‐
tax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
built Sun's Java package, and PCRE copied it from there. It ultimately
found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain sim‐
ple pattern constructs. For example, the sequence A+B is treated as
A++B because there is no point in backtracking into a sequence of A's
when B must follow.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-
digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the
string can be divided between the internal \D+ repeat and the external
* repeat in a large number of ways, and all have to be tried. (The
example uses [!?] rather than a single character at the end, because
both PCRE and Perl have an optimization that allows for fast failure
when a single character is used. They remember the last single charac‐
ter that is required for a match, and fail early if it is not present
in the string.) If the pattern is changed so that it uses an atomic
group, like this:
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing sub‐
pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10,
it is always taken as a back reference, and causes an error only if
there are not that many capturing left parentheses in the entire pat‐
tern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 10. A "forward back
reference" of this type can make sense when a repetition is involved
and the subpattern to the right has participated in an earlier itera‐
tion.
It is not possible to have a numerical "forward back reference" to a
subpattern whose number is 10 or more using this syntax because a
sequence such as \50 is interpreted as a character defined in octal.
See the subsection entitled "Non-printing characters" above for further
details of the handling of digits following a backslash. There is no
such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
Another way of avoiding the ambiguity inherent in the use of digits
following a backslash is to use the \g escape sequence. This escape
must be followed by an unsigned number or a negative number, optionally
enclosed in braces. These examples are all identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without the ambigu‐
ity that is present in the older syntax. It is also useful when literal
digits follow the reference. A negative number is a relative reference.
Consider this example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started captur‐
ing subpattern before \g, that is, is it equivalent to \2 in this exam‐
ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
references can be helpful in long patterns, and also in patterns that
are created by joining together fragments that contain references
within themselves.
A back reference matches whatever actually matched the capturing sub‐
pattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a way
of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the back reference, the case of letters is relevant. For exam‐
ple,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
There are several different ways of writing back references to named
subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
unified back reference syntax, in which \g can be used for both numeric
and named references, is also supported. We could rewrite the above
example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name may appear in the pattern
before or after the reference.
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail by default. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". However, if
the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer‐
ence to an unset value matches an empty string.
Because there may be many capturing parentheses in a pattern, all dig‐
its following a backslash are taken as part of a potential back refer‐
ence number. If the pattern continues with a digit character, some
delimiter must be used to terminate the back reference. If the
PCRE_EXTENDED option is set, this can be white space. Otherwise, the
\g{ syntax or an empty comment (see "Comments" below) can be used.
Recursive back references
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated sub‐
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter‐
ation of the subpattern, the back reference matches the character
string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need
to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
Back references of this type cause the group that they reference to be
treated as an atomic group. Once the whole group has been matched, a
subsequent matching failure cannot cause backtracking into the middle
of the group.
ASSERTIONS
An assertion is a test on the characters following or preceding the
current matching point that does not actually consume any characters.
The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
described above.
More complicated assertions are coded as subpatterns. There are two
kinds: those that look ahead of the current position in the subject
string, and those that look behind it. An assertion subpattern is
matched in the normal way, except that it does not cause the current
matching position to be changed.
Assertion subpatterns are not capturing subpatterns. If such an asser‐
tion contains capturing subpatterns within it, these are counted for
the purposes of numbering the capturing subpatterns in the whole pat‐
tern. However, substring capturing is carried out only for positive
assertions. (Perl sometimes, but not always, does do capturing in nega‐
tive assertions.)
For compatibility with Perl, assertion subpatterns may be repeated;
though it makes no sense to assert the same thing several times, the
side effect of capturing parentheses may occasionally be useful. In
practice, there only three cases:
(1) If the quantifier is {0}, the assertion is never obeyed during
matching. However, it may contain internal capturing parenthesized
groups that are called from elsewhere via the subroutine mechanism.
(2) If quantifier is {0,n} where n is greater than zero, it is treated
as if it were {0,1}. At run time, the rest of the pattern match is
tried with and without the assertion, the order depending on the greed‐
iness of the quantifier.
(3) If the minimum repetition is greater than zero, the quantifier is
ignored. The assertion is obeyed just once when encountered during
matching.
Lookahead assertions
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the semi‐
colon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the
most convenient way to do it is with (?!) because an empty string
always matches, so an assertion that requires there not to be an empty
string must always fail. The backtracking control verb (*FAIL) or (*F)
is a synonym for (?!).
Lookbehind assertions
Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The
contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are sev‐
eral top-level alternatives, they do not all have to have the same
fixed length. Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different length
strings are permitted only at the top level of a lookbehind assertion.
This is an extension compared with Perl, which requires all branches to
match the same length of string. An assertion such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can match two
different lengths, but it is acceptable to PCRE if rewritten to use two
top-level branches:
(?<=abc|abde)
In some cases, the escape sequence \K (see above) can be used instead
of a lookbehind assertion to get round the fixed-length restriction.
The implementation of lookbehind assertions is, for each alternative,
to temporarily move the current position back by the fixed length and
then try to match. If there are insufficient characters before the cur‐
rent position, the assertion fails.
In a UTF mode, PCRE does not allow the \C escape (which matches a sin‐
gle data unit even in a UTF mode) to appear in lookbehind assertions,
because it makes it impossible to calculate the length of the lookbe‐
hind. The \X and \R escapes, which can match different numbers of data
units, are also not permitted.
"Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
lookbehinds, as long as the subpattern matches a fixed-length string.
Recursion, however, is not supported.
Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching of fixed-length strings at the
end of subject strings. Consider a simple pattern such as
abcd$
when applied to a long string that does not match. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject
and then see if what follows matches the rest of the pattern. If the
pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
last character, then all but the last two characters, and so on. Once
again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only the
entire string. The subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails immediately.
For long strings, this approach makes a significant difference to the
processing time.
Using multiple assertions
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice that
each of the assertions is applied independently at the same point in
the subject string. First there is a check that the previous three
characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" pre‐
ceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it doesn't match "123abc‐
foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern con‐
ditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a specific capturing subpat‐
tern has already been matched. The two possible forms of conditional
subpattern are:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alterna‐
tives in the subpattern, a compile-time error occurs. Each of the two
alternatives may itself contain nested subpatterns of any form, includ‐
ing conditional subpatterns; the restriction to two alternatives
applies only at the level of the condition. This pattern fragment is an
example where the alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are four kinds of condition: references to subpatterns, refer‐
ences to recursion, a pseudo-condition called DEFINE, and assertions.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence of digits,
the condition is true if a capturing subpattern of that number has pre‐
viously matched. If there is more than one capturing subpattern with
the same number (see the earlier section about duplicate subpattern
numbers), the condition is true if any of them have matched. An alter‐
native notation is to precede the digits with a plus or minus sign. In
this case, the subpattern number is relative rather than absolute. The
most recently opened parentheses can be referenced by (?(-1), the next
most recent by (?(-2), and so on. Inside loops it can also make sense
to refer to subsequent groups. The next parentheses to be opened can be
referenced as (?(+1), and so on. (The value zero in any of these forms
is not used; it provokes a compile-time error.)
Consider the following pattern, which contains non-significant white
space to make it more readable (assume the PCRE_EXTENDED option) and to
divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The sec‐
ond part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether or not the
first set of parentheses matched. If they did, that is, if subject
started with an opening parenthesis, the condition is true, and so the
yes-pattern is executed and a closing parenthesis is required. Other‐
wise, since no-pattern is not present, the subpattern matches nothing.
In other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
If you were embedding this pattern in a larger one, you could use a
relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the larger
pattern.
Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
used subpattern by name. For compatibility with earlier versions of
PCRE, which had this facility before Perl, the syntax (?(name)...) is
also recognized.
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test
is applied to all subpatterns of the same name, and is true if any one
of them has matched.
Checking for pattern recursion
If the condition is the string (R), and there is no subpattern with the
name R, the condition is true if a recursive call to the whole pattern
or any subpattern has been made. If digits or a name preceded by amper‐
sand follow the letter R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into a subpattern
whose number or name is given. This condition does not check the entire
recursion stack. If the name used in a condition of this kind is a
duplicate, the test is applied to all subpatterns of the same name, and
is true if any one of them is the most recent recursion.
At "top level", all these recursion test conditions are false. The
syntax for recursive patterns is described below.
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), and there is no subpattern
with the name DEFINE, the condition is always false. In this case,
there may be only one alternative in the subpattern. It is always
skipped if control reaches this point in the pattern; the idea of
DEFINE is that it can be used to define subroutines that can be refer‐
enced from elsewhere. (The use of subroutines is described below.) For
example, a pattern to match an IPv4 address such as "192.168.23.245"
could be written like this (ignore white space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which a another
group named "byte" is defined. This matches an individual component of
an IPv4 address (a number less than 256). When matching takes place,
this part of the pattern is skipped because DEFINE acts like a false
condition. The rest of the pattern uses references to the named group
to match the four dot-separated components of an IPv4 address, insist‐
ing on a word boundary at each end.
Assertion conditions
If the condition is not in any of the above formats, it must be an
assertion. This may be a positive or negative lookahead or lookbehind
assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other words,
it tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative;
otherwise it is matched against the second. This pattern matches
strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
COMMENTS
There are two ways of including comments in patterns that are processed
by PCRE. In both cases, the start of the comment must not be in a char‐
acter class, nor in the middle of any other sequence of related charac‐
ters such as (?: or a subpattern name or number. The characters that
make up a comment play no part in the pattern matching.
The sequence (?# marks the start of a comment that continues up to the
next closing parenthesis. Nested parentheses are not permitted. If the
PCRE_EXTENDED option is set, an unescaped # character also introduces a
comment, which in this case continues to immediately after the next
newline character or character sequence in the pattern. Which charac‐
ters are interpreted as newlines is controlled by the options passed to
a compiling function or by a special sequence at the start of the pat‐
tern, as described in the section entitled "Newline conventions" above.
Note that the end of this type of comment is a literal newline sequence
in the pattern; escape sequences that happen to represent a newline do
not count. For example, consider this pattern when PCRE_EXTENDED is
set, and the default newline convention is in force:
abc #comment \n still comment
On encountering the # character, pcre_compile() skips along, looking
for a newline in the pattern. The sequence \n is still literal at this
stage, so it does not terminate the comment. Only an actual character
with the code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth.
For some time, Perl has provided a facility that allows regular expres‐
sions to recurse (amongst other things). It does this by interpolating
Perl code in the expression at run time, and the code can refer to the
expression itself. A Perl pattern using code interpolation to solve the
parentheses problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case
refers recursively to the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead,
it supports special syntax for recursion of the entire pattern, and
also for individual subpattern recursion. After its introduction in
PCRE and Python, this kind of recursion was subsequently introduced
into Perl at release 5.10.
A special item that consists of (? followed by a number greater than
zero and a closing parenthesis is a recursive subroutine call of the
subpattern of the given number, provided that it occurs inside that
subpattern. (If not, it is a non-recursive subroutine call, which is
described in the next section.) The special item (?R) or (?0) is a
recursive call of the entire regular expression.
This PCRE pattern solves the nested parentheses problem (assume the
PCRE_EXTENDED option is set so that white space is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a
recursive match of the pattern itself (that is, a correctly parenthe‐
sized substring). Finally there is a closing parenthesis. Note the use
of a possessive quantifier to avoid backtracking into sequences of non-
parentheses.
If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
( \( ( [^()]++ | (?1) )* \) )
We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be
tricky. This is made easier by the use of relative references. Instead
of (?1) in the pattern above you can write (?-2) to refer to the second
most recently opened parentheses preceding the recursion. In other
words, a negative number counts capturing parentheses leftwards from
the point at which it is encountered.
It is also possible to refer to subsequently opened parentheses, by
writing references such as (?+2). However, these cannot be recursive
because the reference is not inside the parentheses that are refer‐
enced. They are always non-recursive subroutine calls, as described in
the next section.
An alternative approach is to use named parentheses instead. The Perl
syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
supported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the earliest
one is used.
This particular example pattern that we have been looking at contains
nested unlimited repeats, and so the use of a possessive quantifier for
matching strings of non-parentheses is important when applying the pat‐
tern to strings that do not match. For example, when this pattern is
applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a possessive quantifier is
not used, the match runs for a very long time indeed because there are
so many different ways the + and * repeats can carve up the subject,
and all have to be tested before failure can be reported.
At the end of a match, the values of capturing parentheses are those
from the outermost level. If you want to obtain intermediate values, a
callout function can be used (see below and the pcrecallout documenta‐
tion). If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is "ef",
which is the last value taken on at the top level. If a capturing sub‐
pattern is not matched at the top level, its final captured value is
unset, even if it was (temporarily) set at a deeper level during the
matching process.
If there are more than 15 capturing parentheses in a pattern, PCRE has
to obtain extra memory to store data during a recursion, which it does
by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
Do not confuse the (?R) item with the condition (R), which tests for
recursion. Consider this pattern, which matches text in angle brack‐
ets, allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permit‐
ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with
two different alternatives for the recursive and non-recursive cases.
The (?R) item is the actual recursive call.
Differences in recursion processing between PCRE and Perl
Recursion processing in PCRE differs from Perl in two important ways.
In PCRE (like Python, but unlike Perl), a recursive subpattern call is
always treated as an atomic group. That is, once it has matched some of
the subject string, it is never re-entered, even if it contains untried
alternatives and there is a subsequent matching failure. This can be
illustrated by the following pattern, which purports to match a palin‐
dromic string that contains an odd number of characters (for example,
"a", "aba", "abcba", "abcdcba"):
^(.|(.)(?1)\2)$
The idea is that it either matches a single character, or two identical
characters surrounding a sub-palindrome. In Perl, this pattern works;
in PCRE it does not if the pattern is longer than three characters.
Consider the subject string "abcba":
At the top level, the first character is matched, but as it is not at
the end of the string, the first alternative fails; the second alterna‐
tive is taken and the recursion kicks in. The recursive call to subpat‐
tern 1 successfully matches the next character ("b"). (Note that the
beginning and end of line tests are not part of the recursion).
Back at the top level, the next character ("c") is compared with what
subpattern 2 matched, which was "a". This fails. Because the recursion
is treated as an atomic group, there are now no backtracking points,
and so the entire match fails. (Perl is able, at this point, to re-
enter the recursion and try the second alternative.) However, if the
pattern is written with the alternatives in the other order, things are
different:
^((.)(?1)\2|.)$
This time, the recursing alternative is tried first, and continues to
recurse until it runs out of characters, at which point the recursion
fails. But this time we do have another alternative to try at the
higher level. That is the big difference: in the previous case the
remaining alternative is at a deeper recursion level, which PCRE cannot
use.
To change the pattern so that it matches all palindromic strings, not
just those with an odd number of characters, it is tempting to change
the pattern to this:
^((.)(?1)\2|.?)$
Again, this works in Perl, but not in PCRE, and for the same reason.
When a deeper recursion has matched a single character, it cannot be
entered again in order to match an empty string. The solution is to
separate the two cases, and write out the odd and even cases as alter‐
natives at the higher level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
If you want to match typical palindromic phrases, the pattern has to
ignore all non-word characters, which can be done like this:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
If run with the PCRE_CASELESS option, this pattern matches phrases such
as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
Perl. Note the use of the possessive quantifier *+ to avoid backtrack‐
ing into sequences of non-word characters. Without this, PCRE takes a
great deal longer (ten times or more) to match typical phrases, and
Perl takes so long that you think it has gone into a loop.
WARNING: The palindrome-matching patterns above work only if the sub‐
ject string does not start with a palindrome that is shorter than the
entire string. For example, although "abcba" is correctly matched, if
the subject is "ababa", PCRE finds the palindrome "aba" at the start,
then fails at top level because the end of the string does not follow.
Once again, it cannot jump back into the recursion to try other alter‐
natives, so the entire match fails.
The second way in which PCRE and Perl differ in their recursion pro‐
cessing is in the handling of captured values. In Perl, when a subpat‐
tern is called recursively or as a subpattern (see the next section),
it has no access to any values that were captured outside the recur‐
sion, whereas in PCRE these values can be referenced. Consider this
pattern:
^(.)(\1|a(?2))
In PCRE, this pattern matches "bab". The first capturing parentheses
match "b", then in the second group, when the back reference \1 fails
to match "b", the second alternative matches "a" and then recurses. In
the recursion, \1 does now match "b" and so the whole match succeeds.
In Perl, the pattern fails to match because inside the recursive call
\1 cannot access the externally set value.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by
name) is used outside the parentheses to which it refers, it operates
like a subroutine in a programming language. The called subpattern may
be defined before or after the reference. A numbered reference can be
absolute or relative, as in these examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the other
two strings. Another example is given in the discussion of DEFINE
above.
All subroutine calls, whether recursive or not, are always treated as
atomic groups. That is, once a subroutine has matched some of the sub‐
ject string, it is never re-entered, even if it contains untried alter‐
natives and there is a subsequent matching failure. Any capturing
parentheses that are set during the subroutine call revert to their
previous values afterwards.
Processing options such as case-independence are fixed when a subpat‐
tern is defined, so if it is used as a subroutine, such options cannot
be changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
an alternative syntax for referencing a subpattern as a subroutine,
possibly recursively. Here are two of the examples used above, rewrit‐
ten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:
(abc)(?i:\g<-1>)
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
synonymous. The former is a back reference; the latter is a subroutine
call.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub‐
strings that match the same pair of parentheses when there is a repeti‐
tion.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides
an external function by putting its entry point in the global variable
pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit
library). By default, this variable contains NULL, which disables all
calling out.
Within a regular expression, (?C) indicates the points at which the
external function is to be called. If you want to identify different
callout points, you can put a number less than 256 after the letter C.
The default value is zero. For example, this pattern has two callout
points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call‐
outs are automatically installed before each item in the pattern. They
are all numbered 255. If there is a conditional group in the pattern
whose condition is an assertion, an additional callout is inserted just
before the condition. An explicit callout may also be set at this posi‐
tion, as in this example:
(?(?C9)(?=a)abc|def)
Note that this applies only to assertion conditions, not to other types
of condition.
During matching, when PCRE reaches a callout point, the external func‐
tion is called. It is provided with the number of the callout, the
position in the pattern, and, optionally, one item of data originally
supplied by the caller of the matching function. The callout function
may cause matching to proceed, to backtrack, or to fail altogether.
By default, PCRE implements a number of optimizations at compile time
and matching time, and one side-effect is that sometimes callouts are
skipped. If you need all possible callouts to happen, you need to set
options that disable the relevant optimizations. More details, and a
complete description of the interface to the callout function, are
given in the pcrecallout documentation.
BACKTRACKING CONTROL
Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
which are still described in the Perl documentation as "experimental
and subject to change or removal in a future version of Perl". It goes
on to say: "Their usage in production code should be noted to avoid
problems during upgrades." The same remarks apply to the PCRE features
described in this section.
The new verbs make use of what was previously invalid syntax: an open‐
ing parenthesis followed by an asterisk. They are generally of the form
(*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
differently depending on whether or not a name is present. A name is
any sequence of characters that does not include a closing parenthesis.
The maximum length of name is 255 in the 8-bit library and 65535 in the
16-bit and 32-bit libraries. If the name is empty, that is, if the
closing parenthesis immediately follows the colon, the effect is as if
the colon were not there. Any number of these verbs may occur in a
pattern.
Since these verbs are specifically related to backtracking, most of
them can be used only when the pattern is to be matched using one of
the traditional matching functions, because these use a backtracking
algorithm. With the exception of (*FAIL), which behaves like a failing
negative assertion, the backtracking control verbs cause an error if
encountered by a DFA matching function.
The behaviour of these verbs in repeated groups, assertions, and in
subpatterns called as subroutines (whether or not recursively) is docu‐
mented below.
Optimizations that affect backtracking verbs
PCRE contains some optimizations that are used to speed up matching by
running some checks at the start of each match attempt. For example, it
may know the minimum length of matching subject, or that a particular
character must be present. When one of these optimizations bypasses the
running of a match, any included backtracking verbs will not, of
course, be processed. You can suppress the start-of-match optimizations
by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com‐
pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
There is more discussion of this option in the section entitled "Option
bits for pcre_exec()" in the pcreapi documentation.
Experiments with Perl suggest that it too has similar optimizations,
sometimes leading to anomalous results.
Verbs that act immediately
The following verbs act as soon as they are encountered. They may not
be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the remainder
of the pattern. However, when it is inside a subpattern that is called
as a subroutine, only that subpattern is ended successfully. Matching
then continues at the outer level. If (*ACCEPT) in triggered in a posi‐
tive assertion, the assertion succeeds; in a negative assertion, the
assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is cap‐
tured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap‐
tured by the outer parentheses.
(*FAIL) or (*F)
This verb causes a matching failure, forcing backtracking to occur. It
is equivalent to (?!) but easier to read. The Perl documentation notes
that it is probably useful only when combined with (?{}) or (??{}).
Those are, of course, Perl features that are not present in PCRE. The
nearest equivalent is the callout feature, as for example in this pat‐
tern:
a+(?C)(*FAIL)
A match with the string "aaaa" always fails, but the callout is taken
before each backtrack happens (in this example, 10 times).
Recording which path was taken
There is one verb whose main purpose is to track how a match was
arrived at, though it also has a secondary use in conjunction with
advancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There may be as many
instances of (*MARK) as you like in a pattern, and their names do not
have to be unique.
When a match succeeds, the name of the last-encountered (*MARK:NAME),
(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to
the caller as described in the section entitled "Extra data for
pcre_exec()" in the pcreapi documentation. Here is an example of
pcretest output, where the /K modifier requests the retrieval and out‐
putting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this exam‐
ple it indicates which of the two alternatives matched. This is a more
efficient way of obtaining this information than putting each alterna‐
tive in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that is
true, the name is recorded and passed back if it is the last-encoun‐
tered. This does not happen for negative assertions or failing positive
assertions.
After a partial match or a failed match, the last encountered name in
the entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XP
No match, mark = B
Note that in this unanchored example the mark is retained from the
match attempt that started at the letter "X" in the subject. Subsequent
match attempts starting at "P" and then with an empty string do not get
as far as the (*MARK) item, but nevertheless do not reset it.
If you are interested in (*MARK) values after failed matches, you
should probably set the PCRE_NO_START_OPTIMIZE option (see above) to
ensure that the match is always attempted.
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching con‐
tinues with what follows, but if there is no subsequent match, causing
a backtrack to the verb, a failure is forced. That is, backtracking
cannot pass to the left of the verb. However, when one of these verbs
appears inside an atomic group or an assertion that is true, its effect
is confined to that group, because once the group has been matched,
there is never any backtracking into it. In this situation, backtrack‐
ing can "jump back" to the left of the entire atomic group or asser‐
tion. (Remember also, as stated above, that this localization also
applies in subroutine calls.)
These verbs differ in exactly what kind of failure occurs when back‐
tracking reaches them. The behaviour described below is what happens
when the verb is not in a subroutine or an assertion. Subsequent sec‐
tions cover these special cases.
(*COMMIT)
This verb, which may not be followed by a name, causes the whole match
to fail outright if there is a later matching failure that causes back‐
tracking to reach it. Even if the pattern is unanchored, no further
attempts to find a match by advancing the starting point take place. If
(*COMMIT) is the only backtracking verb that is encountered, once it
has been passed pcre_exec() is committed to finding a match at the cur‐
rent starting point, or not at all. For example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a kind
of dynamic anchor, or "I've started, so I must finish." The name of the
most recently passed (*MARK) in the path is passed back when (*COMMIT)
forces a match failure.
If there is more than one backtracking verb in a pattern, a different
one that follows (*COMMIT) may be triggered first, so merely passing
(*COMMIT) during a match does not always guarantee that a match must be
at this starting point.
Note that (*COMMIT) at the start of a pattern is not the same as an
anchor, unless PCRE's start-of-match optimizations are turned off, as
shown in this output from pcretest:
re> /(*COMMIT)abc/
data> xyzabc
0: abc
data> xyzabc\Y
No match
For this pattern, PCRE knows that any match must start with "a", so the
optimization skips along the subject to "a" before applying the pattern
to the first set of data. The match attempt then succeeds. In the sec‐
ond set of data, the escape sequence \Y is interpreted by the pcretest
program. It causes the PCRE_NO_START_OPTIMIZE option to be set when
pcre_exec() is called. This disables the optimization that skips along
to the first character. The pattern is now applied starting at "x", and
so the (*COMMIT) causes the match to fail without trying any other
starting points.
(*PRUNE) or (*PRUNE:NAME)
This verb causes the match to fail at the current starting position in
the subject if there is a later matching failure that causes backtrack‐
ing to reach it. If the pattern is unanchored, the normal "bumpalong"
advance to the next starting character then happens. Backtracking can
occur as usual to the left of (*PRUNE), before it is reached, or when
matching to the right of (*PRUNE), but if there is no match to the
right, backtracking cannot cross (*PRUNE). In simple cases, the use of
(*PRUNE) is just an alternative to an atomic group or possessive quan‐
tifier, but there are some uses of (*PRUNE) that cannot be expressed in
any other way. In an anchored pattern (*PRUNE) has the same effect as
(*COMMIT).
The behaviour of (*PRUNE:NAME) is the not the same as
(*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is
remembered for passing back to the caller. However, (*SKIP:NAME)
searches only for names set with (*MARK).
(*SKIP)
This verb, when given without a name, is like (*PRUNE), except that if
the pattern is unanchored, the "bumpalong" advance is not to the next
character, but to the position in the subject where (*SKIP) was encoun‐
tered. (*SKIP) signifies that whatever text was matched leading up to
it cannot be part of a successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting point
skips on to start the next attempt at "c". Note that a possessive quan‐
tifer does not have the same effect as this example; although it would
suppress backtracking during the first match attempt, the second
attempt would start at the second character instead of skipping on to
"c".
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modified. When it
is triggered, the previous path through the pattern is searched for the
most recent (*MARK) that has the same name. If one is found, the
"bumpalong" advance is to the subject position that corresponds to that
(*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with
a matching name is found, the (*SKIP) is ignored.
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).
(*THEN) or (*THEN:NAME)
This verb causes a skip to the next innermost alternative when back‐
tracking reaches it. That is, it cancels any further backtracking
within the current alternative. Its name comes from the observation
that it can be used for a pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items
after the end of the group if FOO succeeds); on failure, the matcher
skips to the second alternative and tries COND2, without backtracking
into COND1. If that succeeds and BAR fails, COND3 is tried. If subse‐
quently BAZ fails, there are no more alternatives, so there is a back‐
track to whatever came before the entire group. If (*THEN) is not
inside an alternation, it acts like (*PRUNE).
The behaviour of (*THEN:NAME) is the not the same as
(*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is
remembered for passing back to the caller. However, (*SKIP:NAME)
searches only for names set with (*MARK).
A subpattern that does not contain a | character is just a part of the
enclosing alternative; it is not a nested alternation with only one
alternative. The effect of (*THEN) extends beyond such a subpattern to
the enclosing alternative. Consider this pattern, where A, B, etc. are
complex pattern fragments that do not contain any | characters at this
level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does not
backtrack into A; instead it moves to the next alternative, that is, D.
However, if the subpattern containing (*THEN) is given an alternative,
it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. After a
failure in C, matching moves to (*FAIL), which causes the whole subpat‐
tern to fail because there are no more alternatives to try. In this
case, matching does now backtrack into A.
Note that a conditional subpattern is not considered as having two
alternatives, because only one is ever used. In other words, the |
character in a conditional subpattern has a different meaning. Ignoring
white space, consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because .*? is
ungreedy, it initially matches zero characters. The condition (?=a)
then fails, the character "b" is matched, but "c" is not. At this
point, matching does not backtrack to .*? as might perhaps be expected
from the presence of the | character. The conditional subpattern is
part of the single alternative that comprises the whole pattern, and so
the match fails. (If there was a backtrack into .*?, allowing it to
match "b", the match would succeed.)
The verbs just described provide four different "strengths" of control
when subsequent matching fails. (*THEN) is the weakest, carrying on the
match at the next alternative. (*PRUNE) comes next, failing the match
at the current starting position, but allowing an advance to the next
character (for an unanchored pattern). (*SKIP) is similar, except that
the advance may be more than one character. (*COMMIT) is the strongest,
causing the entire match to fail.
More than one backtracking verb
If more than one backtracking verb is present in a pattern, the one
that is backtracked onto first acts. For example, consider this pat‐
tern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the entire
match to fail. However, if A and B match, but C fails, the backtrack to
(*THEN) causes the next alternative (ABD) to be tried. This behaviour
is consistent, but is not always the same as Perl's. It means that if
two or more backtracking verbs appear in succession, all the the last
of them has no effect. Consider this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE)
causes it to be triggered, and its action is taken. There can never be
a backtrack onto (*COMMIT).
Backtracking verbs in repeated groups
PCRE differs from Perl in its handling of backtracking verbs in
repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because the
(*COMMIT) in the second repeat of the group acts.
Backtracking verbs in assertions
(*FAIL) in an assertion has its normal effect: it forces an immediate
backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed with‐
out any further processing. In a negative assertion, (*ACCEPT) causes
the assertion to fail without any further processing.
The other backtracking verbs are not treated specially if they appear
in a positive assertion. In particular, (*THEN) skips to the next
alternative in the innermost enclosing group that has alternations,
whether or not this is within the assertion.
Negative assertions are, however, different, in order to ensure that
changing a positive assertion into a negative assertion changes its
result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg‐
ative assertion to be true, without considering any further alternative
branches in the assertion. Backtracking into (*THEN) causes it to skip
to the next enclosing alternative within the assertion (the normal be‐
haviour), but if the assertion does not have such an alternative,
(*THEN) behaves like (*PRUNE).
Backtracking verbs in subroutines
These behaviours occur whether or not the subpattern is called recur‐
sively. Perl's treatment of subroutines is different in some cases.
(*FAIL) in a subpattern called as a subroutine has its normal effect:
it forces an immediate backtrack.
(*ACCEPT) in a subpattern called as a subroutine causes the subroutine
match to succeed without any further processing. Matching then contin‐
ues after the subroutine call.
(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
cause the subroutine match to fail.
(*THEN) skips to the next alternative in the innermost enclosing group
within the subpattern that has alternatives. If there is no such group
within the subpattern, (*THEN) causes the subroutine match to fail.
SEE ALSOpcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
pcre16(3), pcre32(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 14 June 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE 8.38 14 June 2015 PCREPATTERN(3)
